The present invention relates to an enclosed non-aqueous secondary cell capable of shutting-off the electrical connections within the cell when the temperature and/or internal pressure thereof increase and more particularly to an explosion-proof valve structure used in the non-aqueous secondary cell.
The recent progress in high quality, small-sized and/or portable electronic apparatuses has required the use of a secondary cell having a high energy density, as a power supply means for these electronic apparatuses, instead of the conventional cells such as nickel-cadmium cells and lead storage batteries. Under such circumstances, there have recently been investigated and developed nickel-hydrogen cells which make use of hydrogen absorbing alloys as negative electrode materials and non-aqueous secondary cells which make use of substances capable of absorption and release of a light metal as positive and negative electrode materials, and such cells have already been used in some of these electronic apparatuses. In particular, an enclosed non-aqueous secondary cell which makes use of electrodes capable of absorbing and releasing lithium has a high cell voltage on the order of 3.6 V and a high energy density. Therefore, the size and weight thereof can be reduced. Moreover, the enclosed non-aqueous secondary cell has a low degree of self-discharge and is excellent in cycle life and thus it would be presumed that such a cell is widely used, for the future, as a power source for portable machinery and tools.
In a non-aqueous secondary cell, the cell voltage abnormally increases if a current greater than a predetermined level flows through the cell for some reasons during charging and the internal pressure in the cell correspondingly increases due to decomposition of the electrolyte, i.e., a gas-generation reaction. Moreover, if the overcharge condition is further continued, the rate of the decomposition reaction is accelerated and this leads to an abrupt increase in the cell temperature and in the internal pressure thereof and the cell is finally damaged. Japanese Un-examined Patent Publication (hereunder referred to as "J.P. KOKAI") No. Hei 2-112151 discloses a non-aqueous secondary cell provided with a current-cutoff means which can operate in response to an increase in the internal pressure in the cell as a measure for ensuring safety during such overcharge.
In general, the enclosed secondary cell also causes an increase in the temperature thereof upon formation of a short circuit and/or during overcharge and this, in turn, results in evaporation of the electrolyte accommodated in the cell and hence an increase in the internal pressure thereof. If this condition lasts over a long period of time, the internal pressure of the cell continuously increases, the cell would finally be exploded and tile peripheral devices are correspondingly damaged. For this reason, the cell of this type is equipped with an explosion-proof valve which can externally discharge gases generated within the cell at an instance when the internal pressure of the cell arrives at a predetermined level.
The enclosed non-aqueous secondary cell is likewise provided with an explosion-proof valve, but it has recently been found that although the explosion-proof valve is actuated at a predetermined internal pressure of the cell to externally discharge the gases generated within the cell, the cell temperature still continues increasing and the cell is finally exploded, in particular, during overcharge. It is assumed that this is caused for the following reason. The current continues to flow even after the discharge of gases, the cell temperature correspondingly continues to increase simultaneously with an increase in the internal pressure of the cell, this results in an abnormal reaction such as abrupt decomposition of the electrolyte and active materials and accordingly, the cell causes an abrupt temperature raise.
The cell disclosed in J.P. KOKAI No. Hei 2-112151 comprises, as shown in FIG. 14, an explosion-proof valve 6 having, at the central portion thereof, a projected part protruding towards the side of an electrode group; an insulating stripper 15 having, at the central part thereof, a through hole for passing the projected portion of the valve 6 and positioned in such a manner that the stripper 15 comes in contact with the back face of the valve 6; and a lead plate 4 extended from one of electrode plates of the electrode group and welded to the lower face of the projected portion so as to bridge the gap between the back face of the stripper 15 and the lower face of the projection of the valve 6, which are arranged at a closing lid portion. In this case, if the internal pressure of the cell begins to increase due to, for instance, overcharge and/or formation of a short circuit, the explosion-proof valve 6 causes deformation towards the side opposite to the electrode group and the lead plate 4 welded to the projected portion of the valve 6 is simultaneously peeled off and/or broken at the welded portion in response to the deformation of the explosion-proof valve. Thus, the current path is cut off and the worst case, i.e., explosion of the cell can be prevented before it happens.
First of all, however, when the explosion-proof valve 6 is deformed towards the side opposite to the electrode group in the cell disclosed in the foregoing patent, the lead plate 4 is pulled into the through hole of the insulating stripper 15 and the current path is not always cut off with certainty. Secondly, the lead plate 4 in a floating state may come in contact with the inner wall of an armoring can and may sometimes form a short circuit even if the lead plate 4 is peeled off from the explosion-proof valve 6 and/or broken. Thirdly, the precision of the cutoff of the current path by the explosion-proof valve 6 is greatly affected by that of the weld strength or breaking strength of the lead plate 4 and further the foregoing welded portions are exposed to an electrolyte atmosphere at the side of the electrode group in this structure. Therefore, the foregoing strength is influenced by, for instance, corrosion of the welded portion with time, accordingly the current path is often cut off even when the cell is simply dropped or by a vibrational action or the current path is not cut off even when the internal pressure abnormally increases and correspondingly, the cell is often exploded. Fourthly, the lead plate 4 is welded to the projection of the explosion-proof valve 6, a pinhole is sometimes formed due to insufficient welding and in such a case, the current path cannot be cut off since the explosion-proof valve 6 does not cause deformation even when the internal pressure of the cell increases.
In addition, the cell disclosed in Japanese Un-examined Utility Model Publication No. Hei 5-62956 comprises, as shown in FIG. 15, an explosion-proof valve 6 called rupture disc having, at the central part thereof, a dome-like projection protruding towards the side of a group of electrodes and a lead plate 4 extended from one of electrode plates of the electrode group and electrically connected to the projected portion of the valve 6 at a closing lid portion. In this cell, when the internal pressure begins to increase, the dome-like projection of the valve reverses in the direction opposite to the electrode group and causes considerable deformation. At the same time, the electrical connection between the valve 6 and the lead plate 4 is disconnected and thus the current path is also cut off. In addition, the dome-like projection is partially broken and the gas generated within the cell is accordingly discharged from the cell.
Further the cell disclosed in J.P. KOKAI No. Hei 5-343043 has a structure approximately identical to that disclosed in J.P. KOKAI No. Hei 2-112151, but a metal thin plate or a metal disc 16 lies between a projected portion of an explosion-proof valve 6 and a lead plate 4 as shown in FIG. 16. This cell is designed in such a manner that when the internal pressure of the cell increases, the connection between the projected portion of the valve 6 and the metal thin plate or metal disc 16 is disconnected and the current path is thus cut off.
As will be seen from FIG. 17, the cell disclosed in J.P. KOKAI No. Hei 5-347154 comprises, at a closing lid portion, an explosion-proof valve 6; a metal disc 17 welded to the portion of the valve which causes deformation when the internal pressure of the cell increases from the side of a group of electrodes; a lead plate 4 extended from one of electrode plates of the electrode group and electrically connected to the metal disc 17 on the side facing the electrode group; and a working plate 18 pressed against the explosion-proof valve 6 on the side opposed to the electrode group. In this cell, when the internal pressure of the cell increases due to, for instance, overcharge or short, the explosion-proof valve 6 causes deformation towards the direction opposite to the electrode group while pushing the working plate 18 up, the welded portion between the valve 6 and the metal disc 17 is peeled off and the current path is thus cut off.
As will be seen from FIG. 18, the cell disclosed in J.P. KOKAI No. Hei 5-205727 comprises, at a closing lid portion, a bimetal 19 fitted to the outer periphery of a cap which also serves as a positive or negative electrode; a closing plate electrically connected to the bimetal in the usual condition; and a lead plate 4 extended from one of electrode plates of a group of electrodes and connected to the closing plate on the side facing the electrode group. In this case, when the cell temperature begins to increase due to, for instance, short, the bimetal is put in operation so that the current path is cut off at the connection between the bimetal and the closing plate and thus any abnormal overheating of the cell can be prevented. Moreover, when the cell temperature returns to a normal level, the bimetal recovers its normal state and the current path is correspondingly recovered. Thus, the cell can again be used in the normal condition.
This cell is safe when the cell temperature is raised due to short, but is not necessarily safe when the cell is overcharged. More specifically, the cell voltage gradually increases during repeating the operation and reverse of the bimetal since the cell is reverse type one. In this case, the cell may sometimes be exploded as has been discussed above.
In addition, a method in which lithium carbonate is incorporated into a positive electrode in an amount ranging from 0.5 to 15% by weight has been developed as a measure for sufficiently increasing the internal pressure of a cell during overcharge to thus ensure the function of the foregoing current-cutoff means (J.P. KOKAI No. Hei 4-328278).
Incidentally, recent cordless apparatuses have been required to have a rapid charging ability as a means for improving the handling properties thereof. The rapid charging ability of these cordless apparatuses in turn requires the development of a cell to be installed therein which can be charged within a short time period on the order of one hour or 30 minutes. For this reason, the cell to be installed in these apparatuses must be charged at a high charging current without any trouble. In particular, the charging current increases in proportion to the capacity of cells to be charged. Regarding the overcharge, therefore, a countermeasure should be prepared for large current overcharge. To increase the amount of lithium carbonate to be incorporated into the positive electrode is apt to inhibit damage of cells during overcharge with a high charging current, but it has been found that malfunction such as breakage of a positive electrode during charging or production of a cell occurs when the added amount thereof is not less than 6% by weight and therefore, it is difficult to ensure safety during overcharge at a high charging current, simply by addition of lithium carbonate.